Electric furnace



May 6, 1930. J ASCH 1,757,895

ELECTRIC FURNACE Filed Jan. 5, 1928 2 Sheets-She et 1 a7 INVENTOR ndrew C]: A6cb BY MWWM i124 ATTORNEY Patented May 6, 1930 PATENT OFFICE ANDREW J. ASCH, or l mw Yonx, N. Y.

ELECTRIC FURNACE Application filed January 5, 1928. Serial No. 244,678;

it is well known that a break down of thefurnace refractory wall structure, commonly known as the muflle, often occurs in their operation. These break downs, in a greater percentage of cases coming under observation, have been determined to be due largely to the difference in the co-eflicient of expansion of the resistance element and that of the refractory or fire clay, or, in other words,

' it has been found that in building up heat in one of these furnaces, the platinum resistance element reaches a maximum heating condition before the refractory or heating clay, and in cooling down, the latter is hotter than the former, thus making the expansion and contraction variable, and to a greater degree than the variation caused by the initial difference in the co-efiicients of expansion. A further disadvantage and otherwise serious objection to these types of furnaces, wherein the heatin or resistance element is embedded in the re ractory, is the usual total destruction of the furnace structure by the breaking down of the refractory material,

and the consequent impracticability of re pair or replacement of the main parts .of a furnace. 7

The principal object ofthe invention is, therefore,'to provide for a furnace structure of the class set forth, and one which is of an extremely simplified construction and arrangement of parts, such as will effectively overcome the disadvantages aforesaid, in

a. manner that the life of the furnace will be prolonged for an indefinite period, and the parts thereof will be adapted for quick and easyjl replacement whenever necessary or des1re v A further object of the invention is to provide for a form of heating element which will be readily capable of generating the desired or necessary amount of heat with a much lower resistance characteristic than it' has heretofore been thought possible'to employ for the purpose, and which will have a greater radiating surface for a given cross sectional area, such as will also give a substantial uniform radiation of the heat throughout the effective heating zone of the furnace. 7

Another object of the invention is to provide for a resistance element of the character mentioned, and one which is of a nature to render it unnecessary and undesirable that the same be embedded in the refractory material or fire clay forming the outer muflle of the furnace, and as a consequence where-- of the same can and will be supported within the furnace in a manner that it can readily creep and weave, or otherwise exercise all of its expansion and contraction movements without restriction, and, therefore, without danger of distortion, damage or breakage.

A further object of the invention is ,to provide for a furnace of the type set forth, wherein the several parts of the same are capable of being readily, easy and quickly taken apart for inspection, cleaning or replacement of parts, and thereafter the parts re-assembled into set up operative position with equal facility.

A still further object of the invention is to provide for a furnace as hereinbefore characterized, and one embodying a separate and readily removable inner mufile, which, when-positioned within the outer muflle, actsto house the heating or resistance element between the same and-the latter in a manner to effectively protect the element from exposure to atmosphere, and from any and all possible injury incident to the operation of the furnace.

Another important object of the invention is to form the inner muflle of the furnace from a material, such as silica quartz, or the in cooling down, thus entirely eliminating.

usual refractory repairs to or replacement of the same.

A further object of the invention is to provide for a furnace structure wherein the parts of the same may be readily manufactured and assembled without the initial expense of purchase of the platinum metal for the heating elements therefor, and which expense need not be incurred until a furnace is ready for use, such procedure being made possible by reason of the fact that the heating elements are not incorporated in the refractory parts of the furnaces but are separate therefrom, and may be readily placed in position or removed asoccasion may require.

With the foregoing and other equally important objects and advantages in view, the

invention resides in the certain new and useful comb nation, constructlon and arrangement of parts as will be hereinafter more fully described, set forth in the appended claims, and illustrated in the accompanying drawings, in which:

Figure 1 is a perspective view of a practical embodiment, assembly and mounting of the improved furnace,

Figure 2 is a vertical section taken on the line 22 of Figure 3,

Figure 3 is a vertical longitudinal section taken on the line 3-3 of Figure 2, and

Figure 4 is an enlarged fragmentary deta1l, in perspective, of the furnace support and the mounting therefor. 4

Referring to the drawings, wherein similar characters of reference designate corresponding parts throughout the several views thereof, the preferred embodiment of the invention, as shown therein, comprises an outer mufiie of a refractory material, such as fire clay, or the like, which is formed to provide a flat bottom wall 10, an upper and outer arched or semi-cylindrical wall 11, a vertical back or rear wall 12, and an inner arched or semi-cylindrical wall 13, the latter bGlIlgdlSPOSGd on the bottom wall 10, and centered thereon in uniformly spaced relation with respect to the upper andouter arched wall 11. This inner arched wall 13, however, is preferabl formed of a difi'erent refractory material rom that of the walls 10, 11 and 12, namely alundum, while the space between the same and the outer arched wall 11 is completely filled with a heat resisting or insulating material 14, such as asbestos or thelike.

An inner muflie 15, preferably of silica quartz, which has a zero co-eflicient of expansion, is removably positioned within the space of the inner arched wall 13, and the radius of the curved wall of this inner muflie is only sli htly less than that of the latter, such as wi readily allow for the compara tive free and easy insertion and removal of the iimer'mufiie into and out of operative position. Formed in the bottom'wall 10 of the outer mufile is a pair of spaced parallel slits 16, downwardly through which are projected the opposite ends of a heating or low resistance element 17 which is preferably in the form of a st ip of sheet metal or metal foil, such as platinum, the intermediateportionof which is interposed between the opposed curved surfaces of the arched wall 13 of the outer mufile and the arched wall of the inner muflie 15. v I

As shown in Figures 2 and 3, the inner muffle 15, in addition to its arched or semicylindrical wall structure, is provided with a flat bottom wall 15, which rests directly on the bottom wall 10 of the outer mufile, and a back or rear end wall 15".

15" to abut the back or rear wall 12 of the outer muflle, and the edges of its front open end disposed flush in the plane of the front edges of the walls 10 and 11 of the outer mufile.

-a shape and size'conforming to that of the front open end of the inner muflle, whereby access may be readily had to the interior of the latter.

The outer arched wall 11 of the outer muflle is to be enclosed by a housing or wall 19 of sheet metal, preferably sheet" steel, which, when disposed in proper position, has its forward edge portion extended in a manner to overlap the edge of the front wall 18 of the furnace. This metal housing or wall 19 is to be held in place on the furnace at the upper side thereof by means of a clamping member 20, which is preferably made from a strip of resilient metal, and has its opposite end portions 20 angularly bent downwardly to engage over the opposite ends of the furnace, the forward of the end portions 20 engaging over the removable front end wall 18 to initially retain the latter in position.

The furnace, as thus constructed and arranged, is to be supported in proper position of use in a mounting which comprises a pair of parallel angularly flanged members, each having oneof their flanges 21 supported in horizontal plane above a table or supporting plate 22 on the upper ends of pairs of spaced standards or posts 23. These standards or posts 23 are provided in the form of metal 4 through openings formed in the table or plate 22 in alinement with the bores. In the use The inner mufile 15 is of a length to have this back or rear wall of bolt fa'stenings, the lower ends of the same will be projected below the table or plate 22 to have threaded thereon the 1 securing nuts 25, while the head portions of these fastenings are preferably countersunk into the upper faces ofthe flanges 21.

With the parts of the furnace proper assembled as hereinbefore explained and held in assembled condition by the clamp member or bar 20, the same is placed in position on the supporting flanges 21, and between the vertical flanges 21, the opposite ends of the latter being angularly bent inwardly over the flanges 21, as at 21, to engage at the forward and rear ends of the furnace in a manner to retain the same against endwise displacement. The forward ends of the flangesv 21 are extended beyond the angular portions 21*, as at 21", and are provided with upturned portions, forming lugs 212 at the forward ends of their outer side'edges. The upper edges of these upturned portions or lugs 21 are curved downwardly in a forward direction and merge into the forward end edges of the flange portions 21". A member or plate 26 is extended between and supported on these flange ends 21*, and has its opposite ends turned downwardly to provide flange por tions 27, which engage at the outer side edges of the flange portions 21". Recesses 28 are formed in the front edge member or plate 26, immediately inward of the end flanges 27, and are to be engaged over the lugs 21 of the flange ends 21", whereby an interlockmg engagement obtains between the same to retain the member or plate in position on the latter. The opposite ends'of the rear edge of the member or plate 26 are recessed, as at v 29, to engage over the inturned portions 21 pf the vertlcal flanges 21, and so that-the intermediate edge portion of the member or plate, between the. recesses 29, will snugly engage against the lower edge portion of the front wall 18 of the furnace. The length of the member or plate 26 is such that its end flanges 27 will snugly engage at the outer side edges of the flange portions 21', and in a manner to tension the furnace supports in their entirety toward one another, and thereby exert a clamping action on the opposite 51 es of the furnace structure. Thus, the furnace will be securely sustained inposition, and the opposite ends of the housing or wall 19 will be clamped between the vertical flanges 21 and the arched wall of the outer muflie.

To connect the heating or resistance element 17 in circuit with a suitable source of current supply (not shown), the ends of the same, depending downwardly through the slits 16, in the bottom wall 10 of the outer muflle of the furnace, are to be clamped between the upper ends of a pair of connectors, each of which is preferably formed in three parts 31, 32 and 33, the part 31 being of a length equal to the combined length of the parts 32 and 33, when the latter are disposed in end to end abutting relation, substantially as is shown in Figure 2. Thus, each of the free ends of the heating or resistance element 17 will be clamped between the upper end of the part 31, of a connector, and of the part32, the latter part being secured to the former by. means of screws or the like 34. The opposed faces of the upper end portions of the connector parts 31 and 32 are preferably faced with a thin film or coating of platinum 35, of an area slightly greater than that of the ends of the heating or resistance element 17 to be positioned therebetween. The free ends 36 of the circuit conductors leading from the source of current supply (not shown), are to be similarly clamped between the lower end connector part 31 and the separate lower part 33,-the latter part being secured to the former part by means o f-screws or the like 37. For the purpose of securely clamping the conductor ends 36 in position between the parts 31 and 33 of the connectors,

the opposed faces'of these parts are preferabl whibh open downwardly of the lower ends of the parts to allow for the insertion ofthe conductor ends 36 upwardly thereof for opening and loosening up of the clamping screws 37. When properly assembled, these connectors are supported in position beneath the furnace, and depend through openings 30' formed in the supporting plate or table 22', substantially as is shown in Figures 2 and 3.

As shown in Figures 2 and 3, a thermo cou le 38 will be provided in the rear end of t e inner muflle 15, and is arranged to have its free ends lead downwardlfof the rear wall 15" thereof, and through the rear wall 12 of the outer muflie for connection in circuit with a suitable tem erature indicating instrument or the like not shown) in the usual manner.

From the foregoing, it will be readily apparent that the heating or resistance element 17, although made as it is from fragile metal foil or thin plate, will be as fully housed and otherwise protected from injury by being confined between the inner arched wall 13 of the outer muflle, and the arched wall of the inner muflie 15 as are the heating and resist ance elements of other makes of furnaces,

formedzto provide alined grooves 38,

when the latter are embedded in the refractory material forming the inner wall of such furnaces, and also that, in the event of an injury to or the break down of the foil or sheet form of heating or resistance element 1 17,-the same may be readily removed from position by merely disconnecting its ends from the circuit connectors, and withdrawing the inner muflle 15 from the outer mufile after the furnace has been displaced from its mounting, anda new oneplaced in position. It is also to be noted t at in the case of the accidental break down or fusing of the foil of platinum heating or resistance element 17, or its damage from any other cause, the metal itself is recoverable for use in other dental work, as for instance in crown work, and consequently and by reason of its not being embedded in the refractory material of the inner archedwall of the outer mufiie, its

. loss as the heating or resistance element of the furnace does not result in a total loss of the outer muflle structure or an otherwise serious financial loss, which would ordinarily be sufl'eredby the operator of furnaces of known makes.

It is to be further noted that in the use of a single relatively thin strip of platinum or a sheet of platinum foil of appreciable area for the heating element of the furnace, an

.001 inch in thickness, as is contemplated for use in the present instance of the invention, the strip or sheet of platinum will have'a cross sectional area of .001 square inch, and a radiating area per running inch of 2.002 square inches. Now, comparing the foregoing results with a wire heating element of the same cross sectional area, we find that the wire used would have to be .036 inch in diameter, and would have a radiating surface of only .113 square inch per running inch. This means that a piece of wire of the same cross sectional area, and, therefore, of the same resistance, would only have 1/18 of the radiating surface, and, therefore, would only be ableto impart to the interior of the furnace 1/18 of the amount of heat generated thereby. Now, it will be readily obvious that if the same amount of heat were generated in a piece of wire of the same resistance per inch in length, such a' wire would burn out immediately because it would not be able to radiate its heat fast enough. Again, if a piece of wire were used, which would have the same radiating area or surface per inch and the same resistance as the strip or sheet form of heating element, then that wire would be .093 of an inch in diameter, and it would have to be 6.7 6.inches in length for each inch of strip or sheet form of heating element employed. These latter dimensions of the wire form of resistance element will give the exact foil form of heating element, and the cost of the wire at the present market value of platinum would be approximately forty- 3sfeylen times as much as that of the platinum Without further description, it is thought that the features and advantages of the invention will be readily apparent to those skilled in the art, and it will of course be understood that changes in the form, proportion and minor details of construction may be resorted to, without'departing from the spirit of the invention or,its scope as claimed.

Havin thus fully described the invention,

what is c aimed is: 1. An electrlc furnace comprlslng an outer muffle, an inner muflle removably housed within the outer 'inuflle, a resistance heating element having the form of a relatively thin sheet of platinum of substantially rectangu lar form interposed between the inner of the opposed walls of the outer and inner muflies and having the free end portions thereof projecting outwardly of a wall of the outer muflie, and circuit connectors engaged with the projected end portions of the resistance element.

2. An electric furnace comprising an outer ANDREW J. ASCH.

equivalent. in both radiating surface and in resistance, as compared with the strip or sheet, but this" amount of wire would weigh approximately forty-seven times as much as the equivalent strip or sheet of the platinum 

